what is the best way to merge the double vertices lines in a plane to sphere object without loosing the plane UV map setting?
Use W>Remove doubles to join them. Next, use Ctrl-E>Mark seem along the verts to allow for separation, basically joined edges will always remain connected, messing up your UV map, unless you specify otherwise with mark seem. Finally select everything in the UV image editor and hit P for pin to lock 'em in place. When you UV unwrap again, everything will be in order.